FIG. 1 illustrates an electrical power generating system 10 which has been marketed by the assignee for use in airframes. A plurality of engines, not illustrated, each provide shaft power for driving a three phase alternator which is driven at a constant speed to produce 400 Hz electrical power. The alternator may be driven by a separate constant speed drive (CSD) which has a variable velocity input shaft driven by an aircraft engine and a constant velocity output shaft which drives the three phase alternator or is contained within an integrated drive generator (IDG) which contains a constant speed drive transmission having a variable speed input shaft from the aircraft engine and a constant speed output shaft which drives the three phase alternator. As illustrated in FIG. 1, a plurality of the aforementioned three phase alternators 12 and 14 are provided with it being understood that additional alternators have been used in the prior art. The aforementioned CSD or IDG units have been omitted from the drawing with it being understood that the output 32 labelled "To Speed Control Valve" is an output to a conventional control within an aforementioned CSD or IDG. Each of the alternators 12 and 14 are respectively connected to a power bus 16 to which any number of electrical loads 18 are connected. Because of the common connection of the generators 12 and 14 together through contactor 20 within power bus 16, it is necessary to precisely control the phase and frequency of the alternating current generated by each of the alternators 12 and 14 so that each of the electrical phases produced by generators 12 and 14 are precisely locked in phase and frequency. Generator 12 may function as a master generator which generates alternating current to which alternating current generated by each of the at least one slave generators 14 is phase locked. Alternatively, each of the generators 12 and 14 may function as a slave generator which generates alternating current phase and frequency locked to alternating current generated by an external power source. The bus contactor 20 permits the generators 12 and 14 to be either operated in parallel when the contactor is closed or isolated from each other when the contactor is open. Generator 12 has an associated generator control unit 20 which controls the output shaft velocity of the CSD or IDG by the output 32 to cause electrical current of precisely 400 Hz to be generated. The aforementioned control is conventional. As illustrated, the generator control unit 20 functions as a master unit which provides the reference phase signal over lines 24 to which the generator 14 is phase locked. Generator control unit 22 controls the operation of slave generator 14 in a manner analogous to the control of generator 12 by generator control unit 20. Phase and frequency locking of generator 14 to generator 12 is achieved by the transmission of an analog synchronization signal on the pair of synchronization lines 24 which transmit frequency and phase information for phase locking the generator 14 to the generator 12. In order to insure the highest reliability, an additional pair of lines may be provided which connect the master generator control unit 20 to the slave generator control unit 22 to prevent loss of phase locking by failure of the lines 24. Whichever generator control unit 20 or 22 is assigned the master function also has the functional capability of receiving a signal from an external power source on line 38 to phase lock the alternating current generated by the generators 12 and 14 to a phase of the alternating current generated by the external source. Bus control unit 28 controls the transmission of multiple communications between the generator control units 20 and 22 over serial communication bus 30 in a time multiplexed manner with the aforementioned communications being conventional and forming no part of the present invention. Each of the generator control units 20 and 22 respectively have outputs 32 which are signals respectively applied to a servo-valve control of the aforementioned IDG or CSD controlling the rotational velocity of the shaft driving the generators 12 and 14 to control the frequency of the three-phase alternating current precisely. Load contactors 34 permit the generators 12 and 14 to be respectively disconnected from the loads 18. Contactor 36 permits the power bus 16 to be disconnected from the external power source. Either generator control unit 20 or 22 which is connected to the external power on line 38 provides external phase synchronization information to the other generator control unit by lines 24. As illustrated, the generator control unit 20 is configured to receive an external power source phase reference on line 38 to phase lock both generators 12 and 14 to the external power and alternatively to function as a master control unit when an external power source phase reference is disconnected. Furthermore, independent synchronization of generators 12 and 14, which are not connected in parallel, is useful to avoid beat frequencies.
U.S. Pat. No. 4,308,465 discloses an electrical power generating system having plural electrical power generators operated in parallel which locks the generators in frequency to the average of all of the natural frequencies of the individual generators.
Phase synchronization problems are exacerbated when phase locked generators are physically separated by distances of hundreds of feet of power line such as could occur in a space station.